Many chemicals like N2O, CH4, CO2 have their signature absorption spectra in the mid-infrared (mid-IR) wavelength range. For chemical-sensing based applications, including medical diagnostics, explosive detections and industrial process monitoring, a portable laser emitting in a wide frequency range is desirable. Quantum cascade lasers (QCLs) have been the leading semiconductor laser sources in the mid-IR wavelength range, thanks to the rapid developments in power, wall-plug efficiency, and single mode operation in the last few years. However, the demonstrated frequency tuning either relies on external cavity or has limited wavelength tuning range. Broadband QCLs with heterogeneous active region designs having an extremely broad gain width are useful for mid-IR spectroscopy. Nevertheless, a tunable QCL requires not only a broadband gain medium but also a robust tuning mechanism that allows selecting any wavelength at will in a wide wavelength range.